In the prior art, a variety of devices have been developed for analyzing the components in a gas sample. Among these devices are mass spectrometers and mass spectrographs each of which include three basic components, an ion source, an analyzer and a detector. In operation, a sample gas is introduced into the ion chamber. Once inside the chamber, molecules of the sample gas are ionized by any one of a variety of techniques, such as electron bombardment, photoionization, or surface emission.
The ionized molecules emitted from the chamber are introduced into some form of analyzer which separates the constitutents based on their mass. Since the particles are charged, separation can be achieved through the application of magnetic fields. Other analyzers include electrostatic and quadrupole systems. Once the ions have been separated by mass, they are detected by a sensing means, such as a multistage electron multiplier.
To date, the best known devices are capable of detecting elements in a sample having a concentration as small as a few parts per million. For a large majority of applications, this sensitivity is sufficient. However, in many fields, it is desirable to achieve even greater sensitivity. For example in semiconductor fabrication, in order to maintain high circuit quality and high yields, all gases must be certified at very high purity. More specifically, during epitaxy procedures, impurities, such as hydrocarbons or oxygen can cause formation of pits in the silicon wafer. Other applications which require reliably tested pure gases include air pollution measurements and gas chromatography systems. Accordingly, it would be desirable to provide an apparatus having enhanced sensitivity.
In order to develop an apparatus with enhanced sensitivity, a variety of problems must be overcome. One of the most significant problems relating to the detection of trace elements in a sample concerns the ability to distinguish the measured signals from background noise. As can be appreciated, when the desired signals are only marginally greater than background noise, accurate measurement is inhibited. One method of reducing background noise is to maximize the ratio between the sample gas pressure and the vacuum within the detector. At the present time, the best vacuum or base pressure which can be achieved in the detector is on the order of 10.sup.-8 torr. In contrast, sample gases are typically introduced into the ion chamber at a relatively low pressure of 10.sup.-6 torr. This low pressure is used to prevent the sample gas molecules from interacting with themselves to create dimers and trimers, rather than the monomer ions of interest. Therefore, the ratio between the sample gas pressure (10.sup.-6 torr) and the vacuum condition at the detector (10.sup.-8 torr) is on the order of 10.sup.2. This ratio results in fairly significant signal to noise errors, particularly when attempts are made to isolate trace elements in a parent gas.
Another factor which can adversely affect signal to noise ratio relates to the unwanted detection of electron and photon emissions from the ion source. In many ion sources, ions are created by impacting electrons into the gas molecules. Accordingly, a means must be provided to generate electrons and direct them into contact with the sample. For example, in a typical open bombardment source, a filament is located in contact with the sample. Electrons and photons will be emitted from the filament when it is energized. Frequently, some of these emitted electrons and photons will reach the detector and adversely effect measurement. A peripheral problem associated with this type of open source is that the sample gas tends to contaminate and corrode the filaments. Therefore it would be desirable to provide an improved ion source which reduces electron and photon noise and is less prone to corrosion.
Another difficulty encountered in trace element analysis concerns pressure measurement and regulation. More particularly, the pressure of the sample gas prior to its introduction to the testing apparatus is not constant. Typically, the sample gas pressures will vary from 0.5 atmospheres to 2.0 atmospheres. Therefore, some means must be provided for measuring the input pressure and regulating the pressure of the gas supplied to the ion source.
In the prior art, the pressure measurement problem has been addressed in two different manners, both of which have not proved to be fully satisfactory. One approach is to measure the pressure of the gas before it enters the apparatus. At this stage, when the pressure is in the atmospheric range, gases behave in a viscous manner and can be measured with relatively simple equipment. However, in the viscous condition, the movement or conductance of the sample is dependent on the viscosity of the particular gas. Accordingly, the pressure of the gas received in the ion chamber, downstream from the gas supply, will vary based on its viscosity. Thus, it is difficult to accurately calculate the pressure of the gas sample in the ion chamber based solely on the pressure at the gas supply, since the conductance of each gas sample may be different. Furthermore, the effects of viscosity will become more dominant as the distance between the gas supply and the ion chamber is increased. In the prior art, these measurement inaccuracies were often minimized by calibrating the pressure sensor based on the viscosity of the sample gas introduced into the system. This recalibration, however, is time-consuming and inefficient.
Another measurement approach is to detect the gas pressure after it has been supplied to the ion chamber and reduced to the 10.sup.-6 torr range. At this stage, the gas movement is no longer viscous, but is considered to behave in a molecular flow pattern. This approach eliminates viscous measurement problems. Unfortunately, measurements at the molecular flow level are dependent upon the size of the molecules. Therefore, even in the molecular flow range, instruments must be calibrated to account for the various size molecules in the sample gas. Accordingly, it would be desirable to come up with a pressure measurement and regulation system which minimizes the effect of viscous and molecular flow problems.
Another problem confronted when attempting to measure trace elements in a parent gas relates to the calibration of the detector. More specifically, in this region of measurement, the detector must be very finely tuned in order to provide accurate data. Calibration based on detection of expected trace elements is difficult if not impossible. Therefore, an improvement must be found to facilitate the calibration of the detector to insure accurate measurements of trace elements.
Accordingly, it is an object of the subject invention to provide a new and improved apparatus for detecting trace elements in a sample gas.
It is another object of the subject invention to provide an apparatus for detecting trace elements which includes an improved means for regulating the pressure of the sample gas introduced into the apparatus.
It is a further object of the subject invention to provide an apparatus for detecting trace elements that includes a feedback system for regulating the pressure of the sample gas that is capable of compensating for a wide range of input pressures.
It is still another object of the subject invention to provide an apparatus for detecting trace elements which includes a supply system for introducing the sample gas into the ion chamber at a known pressure.
It is still a further object of the subject invention to provide an apparatus for detecting trace elements which includes a new and improved closed ion source.
It is still another object of the subject invention to provide an apparatus for detecting trace elements which includes a new and improved closed ion source that reduces noise and resists corrosion.
It is still a further object of the subject invention to provide an apparatus for detecting trace elements that includes an improved purge system for clearing the ion source.
It is still another object of the subject invention to provide an apparatus for detecting trace elements which includes an improved calibration means.
It is still a further object of the subject invention to provide an apparatus for detecting trace elements which includes a calibration means that operates by measuring oligomers of the parent gas in order to adjust the scaling of the detector for measuring the trace elements.